Metal extrusion presses are well known in the art, and are used for forming extruded metal products having cross-sectional shapes that generally conform to the shape of the extrusion dies used. A typical metal extrusion press comprises a generally cylindrical container having an outer mantle and an inner tubular liner. The container serves as a temperature controlled enclosure for a billet during extrusion. An extrusion ram is positioned adjacent one end of the container. The end of the extrusion ram abuts a dummy block, which in turn abuts the billet allowing the billet to be advanced through the container. An extrusion die is positioned adjacent the opposite end of the container.
During operation, once the billet is heated to a desired extrusion temperature (typically 800-900° F. for aluminum), it is delivered to the extrusion press. The extrusion ram is then activated to abut the dummy block thereby to advance advancing the billet into the container and towards the extrusion die. Under the pressure exerted by the advancing extrusion ram and dummy block, the billet is extruded through the profile provided in the extrusion die until all or most of the billet material is pushed out of the container, resulting in the extruded product.
In order to attain cost-saving efficiency and productivity in metal extrusion technologies, it is important to achieve thermal alignment of the extrusion press. Thermal alignment is generally defined as the control and maintenance of optimal running temperature of the various extrusion press components. Achieving thermal alignment during production of extruded product ensures that the flow of the extrudable material is uniform, and enables the extrusion press operator to press at a higher speed with less waste.
As will be appreciated, optimal billet temperature can only be maintained if the container can immediately correct any change in the liner temperature during the extrusion process, when and where it occurs. Often all that is required is the addition of relatively small amounts of heat to areas that are deficient.
A number of factors must be considered when assessing the thermal alignment of an extrusion press. For example, the whole of the billet of extrudable material must be at the optimum operating temperature in order to assure uniform flow rates over the cross-sectional area of the billet. The temperature of the liner in the container must also serve to maintain, and not interfere with, the temperature profile of the billet passing therethrough.
Achieving thermal alignment is generally a challenge to an extrusion press operator. During extrusion, the top of the container usually becomes hotter than the bottom. Although conduction is the principal method of heat transfer within the container, radiant heat lost from the bottom surface of the container rises inside the container housing, leading to an increase in temperature at the top. As the front and rear ends of the container are generally exposed, they will lose more heat than the center section of the container. This may result in the center section of the container being hotter than the ends. As well, the temperature at the extrusion die end of the container tends to be slightly higher compared to the ram end, as the billet heats it for a longer period of time. These temperature variations in the container affect the temperature profile of the liner contained therein, which in turn affects the temperature of the billet of extrudable material. The temperature profile of the extrusion die generally conforms to the temperature profile of the liner, and the temperature of the extrusion die affects the flow rate of extrudable material therethrough. Although the average flow rate of extrudable material through the extrusion die is governed by the speed of the ram, flow rates from hotter sections of the billet will be faster compared to cooler sections of the billet. The run-out variance across the cross-sectional profile of a billet can be as great as 1% for every 5° C. difference in temperature. This can adversely affect the shape of the profile of the extruded product. Control of the temperature profiles of the liner and of the container is therefore of great importance to the efficient operation of the extrusion process.
Other challenges arise when the liner passage is non-circular. For example, liners having a rectangular passage, referred to as “rectangular liners”, are used for extruding shapes having generally flat profiles. A conventional approach to forming a rectangular liner involves inserting a pair of liner inserts into an otherwise circular liner. For example, FIGS. 1a and 1b show a prior art container 10 for an extrusion press that comprises a cylindrical mantle 12 and a tubular liner 14 having a passage 16 extending therethrough for receiving a billet. The passage 16 has a generally circular cross-section. A pair of inserts 18 is secured within the interior of the passage by bolts 19 passing through the mantle 12 and the liner 14. Each of the inserts 18 comprises a curved outer surface 18a complimentary to and engaging an inner surface 14a of the liner 14, and a flat inner surface 18b. The flat inner surfaces 18b of the inserts 18 and exposed sections of the inner surface 14a of the liner 14 together define a generally rectangular passage through liner 14, as may be better seen in FIG. 2.
Such “multiple-piece” rectangular liners are known to have drawbacks. For example, stress cracks readily form in the liner 14 along the corners of the rectangular passage during use, which in turn can propagate into the mantle 12, particularly around the vicinity of bolts 19. Additionally, dead metal zones can form at the corners of the rectangular passage, causing metal impurities to be deposited at the corners of the passage. These impurities can be transferred to the extruded product, resulting in an increase in the amount of scrap of extruded product.
Single-piece rectangular liners have been previously considered. For example, U.S. Pat. No. 3,892,114 to Taniguchi et al. discloses a container for use in an extrusion press of the type comprising an inner cylinder having a noncircular opening and an outer cylinder applied about the inner cylinder by shrinkage fit. A plurality of circumferential recesses are provided for the inner or outer cylinder at the interface between them at portions corresponding to the portions of the noncircular opening having small pressure receiving areas. The recess has a predetermined radial depth and extends along the entire axial length of the inner and outer cylinders.
Prior art single-piece rectangular liners typically have high failure rates owing to increased stresses in the liner at the corners of the rectangular passage. This failure typically takes the form of cracking in the liner in the vicinity of these corners. These cracks can eventually propagate from the liner into the mantle, necessitating replacement of both the liner and the mantle and resulting in costly downtime.
Some approaches to resolving the issue of cracking in single-piece rectangular liners have also been previously considered. For example, U.S. Pat. No. 4,007,619 to Ames et al. discloses a container for an extrusion press comprising a liner space having its shape defined by at least one, if desirable several, component parts in which in the liner wall where the highest stresses arise there is provided at least one groove or similar recess. The at least one groove runs approximately in the extrusion direction, and is filled and sealed with a weld which behaves elastically during extrusion. The intrusion of extruded metal into the gaps is thus prevented and a longer lifetime of the container is achieved.
As will be appreciated, improvements to address the problems discussed above are generally desired. It is therefore an object of the present invention at least to provide a novel extrusion press container and a liner for same.